Hearing and casing and guard



April 25, 1950 H. s. KNOWLES 2,505,551

HEARING AID CASING AND GUARD Filed April 19, 1945 s Sheets-Sheet 1INVENTOR. Hus S. KNOWLES S TTORNEY April 1950 H. s. KNOWLES 2,505,551

HEARING AID CASING AND GUARD Filed April 19, 1945 5 Sheets-Sheet 2 IHUGH S,KNOWLES LM W HIS A TTORNEY Fig. IO

' INVENTOR 3 April 25, 1950 H. s. KNOWLES HEARING AID CASING AND GUARD 5Sheets-Sheet 3 Filed April 19, 1945 Fig. II

Fig. 12

- INVENTOR. HUGH S. KNOWLES BY%M Ms A TTORNEY I 4 25, 1950 H. s. KNOWLESHEARING AID CASING AND GUARD April Filed April 19, 1945 I 5 Sheets-Sheet4 Fig./5

,ral

S we M Y E E V 0 N N R K 0 T S r M U S H BY HI Apmfi 25, 195@ H. s.KNOWLES HEARING AID CASING AND GUARD 5 Sheets-Sheet 5 Filed April 19,1945 INVENTOR. HUGH S. KNOWLES H! A TTORNEY Fatented Apr. 25, 1 950 Hugh5.- -Knowle s, River gliorest, {11 11, assi-gnqr to ZenithRadioilorporation, acorporation of .1111- nois Application April 19, 1945,Serial N0."-58-9;134

i 10 Claims.

This. invention relates "to portable hearing aids 0f the type-.adaptedtobe worn-on thepersonoi the user,.usually carriedinlarge, part on orunder ethetclothing of l the user.

The :principal object of the invention is to :eliminate "or reduceinterference in such .a hearing' -aid.

Interference ina hearing aid may. bedeflned asthat portionvoi the; soundoutput whichis' undesired. In general terms itis sound. emitted by s.

the receiver andiheardby-the user of the hearing aid that; is'rdifierentfrom thesoundthat would be heard by a person with normal hearing 'istanding in the same-place and not-usin a hearing aid. Hearing aid;interference may include i soundsduelto (l) Noise generated: in: theimmediate vicinity ofthemicrophone an'dairborne to the microiphonediaphragm or othersensltive unit,

'(2) .Acoustic frequencyumechanical vibrations of the microphone supporttransmitted by the support to the sensitive unit I of :the microphone,and

-(3) -Modulated electromagnetic Waves. picked up by the conductors ofthe hearing aid amplifier circuit and demodulateditherein.

1'0"Gilfib16i3 person having: normal hearingto 'appreciate. theiproblerninvolved'in (l) 1 and. (2) it I may be; pointed :out that thehyper-sensitive area of hearing-cf one usinga hearingaid isxin -.the,;;.

neighborhood of; the microphonewhicheis usually :Worn under or;;adjacent the clothing. -hypeiasensitive-areauof hearing of: one notusing The a hearing aid isthe external: car. which usuallyis I not:covered with "or even; near clothing of any 1 1 sort and such a personis not troubled with; any noise due to clothing rubbing together nearthe earor due tomech'anical? excitation of the outer ear and theadjacent portions of the head.

To. illustrate the point further, a simpleexperi- :ment will bedescribed. If the'finger'. and thumb -are rubbed together withthezhand-near.therwaist the sound willbe scarcely.. perceptible but ifthe hand is'placed near toner ear. the: rubbing aproduces aloudscratching sound. :Ifnow the other hand isused toblock the aural canaloithat ear :the soundzagain-ibecomes imperceptible, the air passage tothe 'ear drum being closed, but if instead of rubbingi fingeri and:thumb together the -fing'eris rubbed onthe outer ear aloud scrapingwgot0 rubbing 0f the 1 ifi adjacent Case noise isagainzheard; this beingdue .tomechanic'al excitation of the head which constitutes thesupport:- for the 'ear- -drum and-otherparts -'of the middle andinner'aear.

I ln-itheigexperimentjust described the airborne the-microphone;supportingcase and the cord 2 noise due =to --rubbing thedinger; andthumb ytO- getherir the neighborhood ofithe ear is analogous tothetairbornemoise, due-to clothingrubbing .toggethennearthemicrophoneeofa hearing aid.

-5 .-"he-.-meehanical excitation-of the head :by rubb IJ -the finger-onthe outer;ear is analogous to the mechanical excitation 'of a,hearinglaidmicrophonecasedue. toclothing rubbing. on. the case or the;cord connected to the case.

:To appreciate the problem involved in-the third :type ofinterferencelistedhereinbefore one need only imagine hewere requiredat5all times to 'wear .a radio headset connected to :v a iradio tuned torqthemostgpowerful localsstation. While radio de- ...15 .tection bya-hearing aid .is not'everywhere a prob- -;-lem,- it Kiss-of. importanceto users; located in the same neighborhood as the transmitter of a radiostation.

-Accordi ng to this invention, the desired-sup- 20.- pressiorroihearingeid interference is obtained by 1) Preventing the generation ofnoise in' the immediate vicinityof. the. microphone that might .,beairborne t0 itSjj diaphragm or sensitive ,por-

tion,

('2) .-a.,-Preven ting the mechanical excitation of connected thereto.-17. Sectionalizing the. support and interposing mechanical vibrationfilters. between the sections. c. Providingdamping for one ;or moreofthe sectionaand (=3) Shieldingthefamplifiercircuit of. the hearing aidfrom unwanted static and radio broadcasts.

Thepreventionof noisegeneration near. the microphone is partly- -aproblem of keeping the microphone away from the. sourceof noise and..par,tly=one;offeliminating the noise. Case-clothing noise isremoved-by. tightly stretchingthe adjacent clothing over the frontofcthemicrophone .to-preventrelative motion-between the cloth and case.or;byproviding-the case-with an outer shell acoustically isolated fromthe -case. InterF-clothing noiseis removed byprovidinga smooth guardnoise duetothe: outer clothing rubbing on the .eguard is kept toaminimum and is generated some -1- distanceaway from-the microphone.

The means;usecl' to prevent airborne'noise due also serves to: preventmechanical excitation of athe: case. iAnyremaining slight vibration ofthe case is kept fromtafiecting the microphone by iproviding a compliantor resistive member be- 5; tween..the:i=microphone and chassis-andbetween the chassis and case. In addition the cord connecting themicrophone case to other parts of the hearing aid apparatus is pluggedinto a separate chassis compliantly or reslstively supported in thecasing and very flexible leads connect the plug sockets to the mainchassis. Furthermore the cord may be dressed along one side of thecasing using compliant clamping means with somewhat slack portionsbetween clamps. Since the vibrations caused by the cord are largelylongitudinal, this is very effective in attenuating the vibrationsbefore they reach the main chassis.

Provision is also made for damping any case vibrations by cementing aninner or outer shell to the casing using a permanently viscous cement.For maximum damping the transverse vibrational impedances of the shelland case should be equal. Thus, if the case is formed of plastic and theshell of metal foil, the foil will be of considerably less thicknessthan the case. It is to be noted that this damping sandwich constructionmay also serve the purpose of isolating the case from adjacent clothingas mentioned before. In this instance the shell should be on the outsideof the case, the case being the member that mechanically supports thechassis, microphone and other interior parts.

If either the case or shell is made of metal sprayed with conductingmaterial, it will serve as a shield to prevent the amplifier circuitfrom picking up electromagnetic radiation. Ihus, in

' the outer case serves as a shield and at the same tune there is noneed to fear the low dissipation qualities of metal since adequatedamping may 'be provided by the cement between the inner and outercases. 4

For a more detailed description of several preferred embodiments of theinvention, reference will be made to the accompanying drawings in whichFigure 1 is a view showing the use of two different forms of acombination spacer-stretcher I unit for stretching the clothing adjacentthe microphone case and spacing outer clothing away from the case;

Figs. 2 and 3 are perspective views to a larger scale of the two unitsshown in Figure 1;

Figs. 4 and 5 are top views of the same units showing also in phantomthe associated microphone case and a section through the pocket carryingthe case;

Fig 6 is a front elevation, partially in section,

. showing the microphone case and the cord con- 7 nection to the case;

Fig. 7 is a detail section on line ll' of Fig. 6

illustrating the manner of dressing the cord along the side of the case;

Fig. 8 is a medial section of a case similar to 4 that shown in Fig. 6showing the manner of mounting the microphone within the casing and alsoshowing one method of damping and isolating the casing by means of anouter shell, the case of Fig. 8 differing from that of Fig. 6 only inthe connector panel mounting, Fig. 8 showing a modified form of suchmounting;

Fig. 9 shows a third form of the spacerstretcher unit illustrated inFigs. 1 through 3 and also shows the manner of use of such units whenworn by a female user instead of a man as previously illustrated;

Fig. 10 is a perspective view to a larger scale of the unit of Fig. 9illustrating its connection with the microphone case;

Fig. 11 is a front view showing another form of the invention in whichthe outer shell isolating the case from adjacent clothing is notcemented to the casing as in Fig. 6 but is instead supported on thecasing by compliant pads at the side of the casing. This view also showsa simple unit for spacing the outer clothing from the casing, this unitalso serving as a clip to hold the case in place;

Fig. 12 is a medial section through the pocket and guard of thearrangement shown in Fig. 11, the hearing aid appearing in sideelevation;

Fig. 13 is a top view of the apparatus of Fig. 11;

Fig. 1% is a top View of a fourth form of spacerstretcher unit similarto the units shown in Figs. 1, 2, 3, 9 and 10 and also shows a modifiedform of case specially adapted to this unit;

Fig. 15 is a view of the inside of the front part of a. third form ofcasing in which the shell is cemented to the inside of the casing andthe microphone is supported on the casing rather than on the chassiswhich carries the amplifier;

Fig. 16 is a medial section through an assembled case of the type shownin Fig. 15;

Fig. 17 is a view of the inside of the back part of the casing of thetype shown in Figs. 15 and 16; and

Fig. 18 is a diagram showing the electric circuit of the completehearing aid.

Referring to Figure 1 there is shown a man wearing two complete hearingaids. Each hearing aid comprises a microphone and amplifier mounted in acasing as shown at l and 2, a set of batteries (not shown), which may beconnected to the microphone and amplifier by means of an electric cordshown at 3 and 4, and an earpiece or telephone receiver adapted to fitinto the external ear as shown at 5 and 6, which is connected to theamplifier by means of an electric cord as at i and B. A man wearing ahearing aid usually carries the casing containing the microphone andamplifier either in his inside coat pocket or in his upper breast pocketas il lustrated in the drawing. In either instance the case containingthe amplifier and microphone is apt to be mechanically excited by thefriction of the pocket cloth rubbing over the exterior of the casing.Likewise there is apt to be a considerable amount of noise generated inthe immediate vicinity of the microphone and airborne thereto due to therubbing of the pocket against adjacent parts of the clothing such as thecoat or vest. The mechanical excitation of the case may be considerablyreduced by immobilizing the cloth of the pocket over the case. This maybe done by clamping a guard member around the pocket and case containedtherein. Such a guard member may take the form shown at 9 in which caseit is completely separate from the case and is supported therefromthrough the cloth of the pocket. In such case the guard member must besufiiciently elastic and must sufficiently surround the casing so as toget a good grip on the casing and pocket. Another method of forming theguard is shown at H]. In this case the guard has a portion ll passing upover the top of the pocket and the casing and having a lug 12 which fitsin a recess in the back part of the casing. In this arrangement the sho1 z a it pointsjof" he? ht ct herew h,o

'1 1 e those WhlCl are a d irt'n l o e. t urefore'i'relyiittletraiismissio chanical vibration fromjhe guard aemea cli-"i'tly to the 'casinahecause of the high im ixedahcebfthe"couplifigbetween these members.

"further pointfofconstruction of both the f trbe's "of guardsillustrated which shouldbe noted is'tli'e "S'p'acihg Between the "guardand the'frcnt "Wall 'of the micr6phb'rie casing. This spacing, asWelles"the'thikhess 'Gfthe guard member itself,

keeps the othercl othifig of "the user away fromthebldth Of the'p'bcketfin which the microphone feasi g "is k placed. This constructionfurther ffis'elates the'"microphonefrom any'noises which'fm'ayfbefienerated due "to-cloth rubbing over the outside of'the'guardmember.

lntth'e'construction shown for the guard mem- "berflVandasbestillustiatecl in Fig. 4,'thesupporting members! I pass over thetopcasing at either'side of the volume control knob i3 and ibetween'this knobxandqthe on-off and tone con- 'J .tro1'switch"1eversM. and 5.The guard member 'thereiore doesjnot, in any way interfere with ;,th enormal use andoperation of the manual con- ;tr ols, associated with theamplifier and micro- "phon ..c i gl a. s. "R errin t ilit. 15 .1 be eemt at th Lenard m mbelr ss r s m a n .constructien to. the 1 gua d.member i 5 xcept h ,inst eds t .sunpqrfiie members e m s dnera tweeasehewnet L6 in l ra q ri leesma and p c t 9 n l i bqt -impe 9f. th u rd9. amt w h r ve iq ribs as shown at I'i' and l8 which bear against ecase through e clothlof the pocket. These relatively ihsehsiti've pointson the casing. he bow-like coristruction of the guard islsuch that whenthe guard 'isin place the bowls slightly ist rai ghtejrlied so, as tostretch the cloth of the beaker; hetween'theribs 11 -1 and 9345.

v "While the ,guams have been described as being ge' ,or1fmo1qeq plastic[they might equally well libe"made'of'ahyjmaterial that is fairlyelastic and has"sufficient.rigidity, for example, plain steel wir zdated' metal springs.

' 7., a d" fshow th C nstru on of iculaifl formbf"aeasing, for the mcrophone d1 am pliflerju it designed to help reduce hearngaicl ihteriereuce. This form of casing may jen b used in a -an e" f h f t i a t 1 teerie-t o clamp the clbtha g'ainst the'casihg connection with a guard ofthe 8 the caeifig ccrhiirises mg clbth a the enter cashie bo tnei i 'fmje q rough a solid yer (Bf-material 2 Fleas been i156; i l 'Subhdificationfhb 'wever, brings t poirlt. lnfallf iristarices it is a ji nep hr oi y iihe flbl hp i ,1 a Jigs over the microphone-face imust beclosely coupled. Furthermore, it is importantto jr'oyifi a" h im dacameam Wat/es lookies h asftheint s aee between the meam lai'gelydeaglair irable ftof'c r mu e $9....5 .7 and thia vthep s i t'a ,e: re du m ei nce due vibration itrarismittegl' to the microphone through 'the cor dd to the'electricicircuit of the microphone ""6 and amplifier but' inthe nore usual construction spec'ai- "connectors age 'firovid'edso thatthe cord "mayjhe 'det'ache'd "from the amplifier and inicrfiiphe iasir'ig. "Irieither' case the' 'external 'co'niu'ctoxjs gamma 'be"meager mo "case aha 7 connected on to'a pariel which is mounted withinshown in Figs. 6 and 8. In Fig. 6, the terminal panel is shown mountedon the outer casing. The panel 22 is shown as being supported in pads ofrubber-like material at 23. Connections from the terminals 24 on thepanel to the terminals 25 of the microphone amplifier circuit are madeby means of the flexible wires 26. This construction provides a maximumisolation of the panel from the chassis 21 supporting the microphone andamplifier. The external conductors 28 are connected to the terminals 24by means of plugs 29. As a further means of preventing transmission ofvibrations from the cord to the microphone the cord may be dressed alongthe side of the casing and clamped thereto at spaced points such as at39 and 3E. As shown in Fig. 7 the clamps comprise spring members 32riveted to the casing at 33 and having rubber pads 34 cemented thereto.The casing is recessed at 35 so that the conductors may be held underthe clamp 34 without slipping out at either side. There should be aslight slack in the cord, as at 36, between the clamps 39 and 3! inorder to further suppress vibrations of the cord. Although the cord 28,comprising in reality two portions, one going to the batteries and theother to the earpiece, has been shown to be dressed along a single sideof the casing, it may be desirable in some cases to dress the batterycable along one side of the casing and the cord going to the earpiecealong another side of the casing. In case a crystal type (highimpedance) earpiece is used, separation of the two cords is particularlydesirable to avoid coupling any stray audio frequency currents in thebattery cord to the cord going to the earpiece.

In Fig. 8 there is shown a slightly modified construction of the panelsupporting the electric connectors. In this instance, the panel 31 issupported at either end on the inner casing by means of elasticcompliant pads 23a similar to the pads 23. Further, in thisconstruction,

the electric connector plugs 38 lie slightly below the surface of thebottom of the outer casing 20 so as to minimize the possibility ofdirect excitation of the plugs themselves. In both the constructionsshown in Figs.'6 and 8 the apertures in the outer casing 29 should besumciently large so that in their equilibrium position the plugs 29 or39 will not touch any adjoining part of the casing.

Referring now to Figs. 9 and there is shown a modified form of theinvention designed for use with female attire. In this form of theinvention the outer guard 39 is attached to the of the case fromexcitation by the clothing of the wearer. Such a top guard isparticularly desirable in the case of a female user since the upperportion of the dress may sag and touch the top portion of the casing ifsuch additional protection is not provided. The guard 39 may be formedof molded plastic or of wire and is in other respects similar to theguards 9 and I9 illustrated in Figs. 1 through 5. It is to be noted thatin this construction in which the upper part of the casing is guarded,the volume, tone and on-ofi control members are located at the frontside of the casing as shown at 43, 44 and 45, these members being moreaccessible there. These members are usually roughened and protrudebeyond the surface of the casing to facilitate operation through a layerof clothing, as when under a dress, for example. Hence they areparticularly susceptible to mechanical excitation. However, the guardmember extends beyond these control members and hence prevents to alarge degree any contact of the control members with the clothing whilestill permitting the clothing to be pushed in against these members whenit is desired to operate them. The guard also prevents undesiredactuation of the control members, especially the volume control, byclothing rubbing against them during normal use of the hearing aid.Previously this has been accomplished only by recessing the controlmembers beneath the surface of the casing, thereby making difficult anyoperation through a layer of clothing.

In Figs. ll, 12 and 13 there is shown another form of the invention inwhich the outer casing 45 protecting the main case against mechanicalexcitation is formed of a sheet of springy material such as metal orplastic and has curved side walls 41 carrying elastic pads 48 which snapinto position in the recessed side walls 49 of the main case. The backwall of casing 46 also carries one or more elastic pads 48a to reduceits response to low frequency vibrations. If desired, the whole casing46 could be made of a grid-like construction similar to that of theouter guard, in which case the additional pads 48a could be dispensedwith. As illustrated, the outer casing 45 surrounds only the back andside walls of the main case, the top, bottom and front walls not beingcovered. However, the outer casing extends suihciently beyond the maincase on all exposed sides so as to prevent to a large extent anyexcitation of these walls. The arrangement is particularly suited foruse in connection with a pocket in which instance the ends 59 of theside Walls of the outer casing stretch the cloth of the pocket out ofcontact with the microphone face of the main case. The outer guard 5| issupported on the outer casing by U-shaped supporting members 52 whichmay constitute extensions of the side members of the outer guard 53 andwhich may be attached to the outer casing at 54. The members 52 extendabove the tone, volume and on-ofi control members, and thus protect themin a manner similar to that provided by portion M of the guard shown inFigs. 9 and 10. The Whole outer casing and outer guard construction maybe such that the outer guard can serve as a clamp to hold the case in;position in the pocket of the wearer. As in otherembodiments the frontwall 55 of the outer guard is spaced at a distance from the most forwardpart of the outer casing and the adjacent cloth of the pocket so as tomore effectively isolate the microphone from any noises generated byfriction of outer clothing rubbing on the outer guard.

In the construction just described the electric cords may be dressedalong the side of the main case underneath the elastic pad 48 or, asillustrated, may be dressed along the inside of the outer casing 4'! andheld in place thereon by clamps 56. The cord should be arranged so as toprovide a slight amount of slack between points where it is clamped tothe outer casing and also a slack loop 51 between the last clamp and the55; mint eiewbeteii mwt he, main case.

when d e in the case With such construction when the 9th f a oe ket t drwn We rOS t e om; a e hQc se between hawed e i e 7 t wi l equt, -wm siti i h. he-i r lithe-ea e inthe imgnegliate vicinity or the microphone.

' r' a nofuthe ca'se firi shou ,be maqeas the amount} offnobingtheteoii.

In Figs. 15 16,. angL 1'ZAtl1t e is shown a, flitther modifieqforinfofc'a'se 'foii the micydphOHQQnd aihpliffiei Ii this 0 r i t n thecase 6 2 is, provided with anihne ca 71 m E3- hich s c m ntd-theretpfwith perm ently qi' us em-eht il b' msi Qim er emem. Bneriandt erh afiine 6,2 and mm Y. beiorme'aefanx e eii, ma erial. but"in a eferrelilfflrm the oiiter casingfiz isa molded plastic andhthe nner1m r B eor ea- 1 1 01 Th h k..- -e o eiq metlendr i ould b v u h that.their ti"ansyer'setimp "ances, e; s ihstam' tialiyequal. In sliich casethe; damping effectpro; vided by the vi,s.c01 1 s. cement, and theinner, liner; s a; x m! t e 'be n edltha mo t mot s oevibration of, thefa e will, due. to th pres: once of the innenl ine 'fii, place, theviscous cement in shea t. This distortion. of the ceme tf produces alarge rd ation of "the xiibra' ti al energy-oi h a efi. m1 m me iereesmie: sion to the micro'hoiie h at 64 si poorteci on the casebyv'iiheans oi. i uhhe ne: 6,55. When the microphfi e is thus; supoqrtgi on, the cas ing', aebeist shown ing'Fig, 16, it is unnecessary toproiriee seiiare iel for s et rt ne t e, electric connectors, 5,6,.These 'cqnnectors may; be mounted, dil' 'ect'ly on the same chassis 611as the tube.

pa s e'iteea nfie'r; Ais'o the roi'l'may be; pier: tielll'f out awaitsat 'H irovi ie' additiohaf space inside the casing. The"c'a's'"cofistifltibn just described may be used effectively ai'oiiebi ihcohih'ihation "with gua such as" illustrated in Figs; '1' "and 9;' 1n;somecas'esitmay also'be desirable topybvide an outer casing asin Figs. 3and 12 although the ihner liner construction of mg. lfi'is in genera} analteith'ative form of protection tot-hat shown n. igs. 8 and 1?. Iidsired, a clip 68 iemovabix attached to, 61th:): the front of. the casin,11 2% electromag eti Waves iriig'ht'bje intero 1-0- Q heb lsoi t e eei1 2 1 meia e q e support t i 'lflq t fiut idbi he '1. clothing ratherthan'ih'a; poeket; Such a clip may be used with. eYiQt h?Pirate 1 .d sed structions/ In all three fo ns a double wall caseconstr ction as shot/in Figs/$12 and 16," i' f"'e"ithei Walt is of metal it will. sefve as anelectric "shield, to

prevent reception' ofjeiectroi nagnetic tweey the conaiuctor's insidethefcasihgQ Apart'iciilarly effective shield illj'he obtained ifhothwalls of, the duaiwailb sandwich typeica sing are made; oi metal.Instead of hiaking one 011 both walls jofi metal either o 'ie 'o bothmay be'sptayed with conducting material" as in kno wh'constr uctiotisjii ordeifto proyici coiiducting shield.

The "shiehii'ngf in 10;; will be better undetstood by an examination: ofthe 'elect io ci'rciii t' of the hearing aid shot/h in Fig. 13;. ThQIILIi u 1 phone comprisesthe pioao c ystaf'lz' which is conneeteci bycbhdoct'oys 13, 14 acioss the 'ihgut of the first amplifier tube i5. 'The outiiut amplifier tube thi's 'cfo'uplejdby condefisefifi andcoi-iductorsfifl; 18', i9, to power tuhe' 3g" Tiihe' fit is coupled byti ansfdi niertlfto terminals 82;831 to which the eafphone afi rhayfb'elcon rie'ted', by leads as, ea, Powe ejmepueeogg the'b'attjetis 3?, 88 toheat the filaments Of the' obesa q'mam; tain the desired poter'it'ialson the'otheflerhehts' of the circuit. The erieg are e'eenebtedtiii theamplifier by meat .ccndglptdis 8,9, 9%; Sit

The conducting sh eld provided by 'the'casing for the microphonefanplifie zuhit is ifidicatd me; by a dashedlin. Were'it not forthisshield} ed b5? either the'coriductors 13, 7.4 cohr 'eotin'g the mfji'crophone and first aihpliiierftubefot the coh'fdiiotoi's iii, E3, E5,couplihg theffi l'st ililbe t6 the power tiihe. Siich wavesar'sbrnetinies demoliulated due to non-linearities of the tubechaifacteristies and appear as inteiference in the eai'ohohe 85,

The condiictihg shield actisjasa Fa aday; cage to preterit thee2;ittenfzeof a riy electric fieljdjwithiii the casing theicebyeliminating this soiiroe "(if interiere ce. It'isi to be noted that theshield also prevents any electi'ic'bha' ges outside the cas ing frominfluencing the potential of the circuit elements Within the casing amithus prevents possible inte fei oi, that type su'ch as mightb'e causedby f ction'alelectrification of; the casing due to clothing'iiibbiiigagainst it.

'Oneother function of the coiidiict'ing shield may be mentioned. Byconnecting the shield to the gositiire side of the plate: supply battery88 dicat'jegi atfiga, t cojn ciudtb'rs 0f the am.-

c chit ate n? e hegative with respect 4 'ght'leakage current will sagainst corrosion at the This connection is par; tropic climates wherethe M and inst llation may have i .i f f i'eeist i e d bes ibjctdeteriora-v tion. a f i'gther fir ajutio'n'; the transformerlaminationfs 33' may "also be connected, as at 94 ci th hi h si Of h mae iii p 's e t fiy and he sin 9 sferee 8-! 'fii qiceted de he line inFig. 18) may be on'hected t the foil liner by oondi ctirig leafspiflng s3'50; (see Fig. 16.).

It Wiiihowbe' seen that by'the use of this inye tion, hearing aid'ihtefeie ence bi neatly all orm ma b m rked y re ced or' nft e i e imiaet d-Wh jef d m q meiit o t nvention have been ribed for purposes of i1-'lietiieiiq 1U$Wi9i ha ma y m ifi' e he. meat, and t i eee i te eve yWes? Letters Patent all forms of the invention falling within the scopeof the appended claims.

I claim:

1. In a hearing aid, a microphone, a vacuum tube amplifier, a casingelastically and compliantly supporting and at least partially enclosingthe microphone'and amplifier, a metal shield supported by and spacedfrom the casing and at least partially enclosing the microphone andamplifier, a guard member disposed over the casf ing and shield in frontof the microphone and spaced therefrom, said casing, shield, and guardbeing apertured to provide a low impedance acoustic channel to themicrophone, and electric connector supported by the casing but separatedfrom the microphone by elastic compliant means, and spring clampingmeans having compliant pads to hold an electric cord at spaced pointsalong the casing.

2. In a hearing aid, a microphone, a vacuum tube amplifier, a casingsubstantially enclosing the microphone and amplifier but having anacoustic aperture before the microphone, a metal casing within the firstmentioned casing to a large extent enclosing the amplifier, saidmicrophone and amplifier each being supported upon one of said casings,a rubber-like filling spacing said casings from each other, and a guardsupported by one of the casings and disposed over the outer casing infront of but spaced from 1 said acoustic aperture therein.

3. In a hearing aid, a microphone, a vacuum tube amplifier, a metalcasing substantially enclosing the microphone and amplifier but havingan acoustic aperture overlying the microphone, a chassis mounted in saidcasing on compliant elastic supports, said microphone and amplifierbeing mounted on the chassis, a panel mounted in said casing oncompliant elastic supports, an electric connector mounted on the panel,flexible leads interconnecting said connector and the amplifier, anouter shell surrounding and substantially enclosing the casing buthaving an acoustic aperture overlying the microphone and having anopening over the electric connector large enough to admit anotherelectric connector without touching said outer shell when the connectoron the panel is in its equilibrium position, a smooth acousticallypervious guard afiixed to the shell and overlying but spaced from theshell near the microphone, and means including a plurality of compliantpads for holding an electric cord along one side of the hearing aid.

In a hearing aid, a microphone, a vacuum tube amplifier, a first metalcasing supporting and substantially enclosing the microphone andamplifier, and a second metal casing surrounding the first metal casing,said both sides of the first casing being recessed and the adjacentsides of the outer casing being conjugately shaped, said outer casingbeing of spring-like construction adapted to snap over the first casingwith said recessed and adjacent sides interlocked.

5. In a hearing aid adapted to be worn in contact with a portion of awearers clothing: a case including an acoustic aperature; and a guardincluding a portion for engaging said case and cooperable therewith toform a clamp for hold ing a portion of said wearers clothing immobilerelative to said case and relative to said acoustic aperture, andfurther including a grilled shield portion of bowed configurationsubtending said immobilized clothing portion and projecting away fromsaid case and said immobilized clothing portion when said guard ispositioned on said case to prevent other clothing portions from comtacting said immobilized portion.

6. In a hearing aid adapted to be worn in contact with a portion of awearers clothing: a case including an acoustic aperture; and a guardin-' cluding clamping portions for engaging said case at points remoteto said acoustic aperture for supporting said guard on said case,including other portions cooperating with said case to form a clamp forholding a portion of said wearers clothing immobile relative to saidcase and relative to said acoustic aperture, and further including agrilled shield portion of bowed con-- figuration subtending saidacoustic aperture and projecting away from said case when said guard ispositioned on said case to shield said case from other clothing of thewearer.

7. In a hearing aid adapted to be worn in contact with a portion of awearers clothing: a case including an acoustic aperture; and a guardincluding parallel clamping surfaces for engaging said case at oppositesides of said acoustic aperture and cooperable with said case to form' aclamp for holding a portion of said wearers clothing immobile relativeto said case and relative to said acoustic aperture, and furtherincluding a grilled shield portion of bowed configuration subtendingsaid immobilized clothing portion and projecting away from said case toprevent other clothing portions from contacting said immobilizedportion.

8. In a hearing aid adapted to be worn in contact with a portion of awearers clothing: a case including an acoustic aperture; and a resilientguard including cooperating jaw sections spaced apart by a distance lessthan the dimension of said case in a direction transverse to saidacoustic aperture, said jaw sections and said case constitutin a clampfor supporting said case on a portion of said wearers clothing, and forholding said portion immobile relative to said case and relative to saidacoustic aperture, and said guard further including a grilled shieldportion of bowed configuration subtending said immobilized clothingportion and projecting away from said case when said guard is positionedon said case to prevent other clothing portions from contacting saidimmobilized portion.

9. In a hearing aid adapted to be worn in contact with a portion of awearers clothing: a case including an acoustic aperture; and a guardincluding a portion for engaging said case and cooperable therewith toform a clamp for bolding a portion of said wearers clothing immobilerelative to said case and relative to said acoustic aperture, furtherincluding a grilled shield portion subtending said immobilized clothingportion and projecting away from said case when said guard is positionedon said case to prevent other clothing portions from contacting saidimmobilized portion, and said guard still further including a secondshield portion projecting away from said case at the top thereof whensaid hearing aid is worn to prevent articles of clothing from contactingsaid top.

10. In a hearing aid adapted to be worn in contact with a portion of awearers clothing and in cluding a case having an acoustic aperture, a

' guard comprising: a pair of clamping surfaces for engaging said caseat opposite sides of said acoustic aperture to support said guard onsaid case; and at least one elongated shield member supported at eachend by one of said clamping surfaces and projecting away from said casewhen '13 said guard is positioned on said case to prevent other clothingportions from contacting said case,

HUGH S. KNOWLES.

REFERENCES CITED UNITED STATES PATENTS Name Date Booth et a1 June 10,1924 Number Number 14 Name Date Weinberger Sept. 6, 1932 Spotts Mar. 14,1933 Olson Jan. 8, 1935 Ballantine July 4, 1939 Kiernan Dec. 5, 1939Wengel Aug. 19, 1941 Shapiro Aug. 17, 1943 Zarth Dec. 14, 1943 CarlisleJune 13, 1944

